Respiratory apparatuses commonly have the ability to alter the humidity of the breathable gas in order to reduce drying of the patient's airway and consequent patient discomfort and associated complications. The use of a humidifier placed between the flow generator and the patient mask produces humidified gas that minimizes drying of the nasal mucosa and increases patient airway comfort. In addition in cooler climates, warm air applied generally to the face area in and about the mask is more comfortable than cold air.
Many humidifier types are available, although the most convenient form is one that is either integrated with or configured to be coupled to the relevant respiratory apparatus. While passive humidifiers can provide some relief, generally a heated humidifier is required to provide sufficient humidity and temperature to the air so that the patient will be comfortable. Humidifiers typically comprise a water tub having a capacity of several hundred milliliters, a heating element for heating the water in the tub, a control to enable the level of humidification to be varied, a gas inlet to receive gas from the flow generator, and a gas outlet adapted to be connected to a patient conduit that delivers the humidified gas to the patient's mask.
Typically, the heating element is incorporated in a heater plate which sits under, and is in thermal contact with, the water tub.
The humidified air may cool on its path along the conduit from the humidifier to the patient, leading to the phenomenon of “rain-out”, or condensation, forming on the inside of the conduit. To counter this, it is known to additionally heat the gas being supplied to the patient by means of a heated wire circuit inserted into the patient conduit which supplies the humidified gas from the humidifier to the patient's mask. Such a system is illustrated in Mosby's Respiratory Care Equipment (7th edition) at page 97. Alternatively the heating wire circuit may be located in the wall of the patient conduit. Such a system is described in U.S. Pat. No. 6,918,389.
In a hospital environment, the ambient temperature of the atmosphere within the hospital is controlled by air conditioning to be generally constant at about, for example 23° C. The required temperature for the humidified gases supplied by the respiratory apparatus may thus be controlled within set temperature parameters. The controlled temperature parameters ensure that the humidified gases are held at a temperature above their dew point to prevent condensation within the breathing circuit.
Humidifiers are often used in a home care environment for use such as in the treatment of breathing and sleep apnea disorders. Humidification systems used with CPAP devices for home use have been limited due to pricing constraints, and by the need to maintain the systems small and lightweight, with a comfortable hose and mask, and a low complexity for untrained users. In systems used in clinics or hospitals, such constraints are generally not an issue and temperature and humidity sensors may be located in the airpath and close to the patient's nose to provide direct feedback to control systems, thus ensuring good performance. The cost, size, weight and discomfort of these systems have not been suited to home use. Home users have therefore relied on experience obtained through trial and error to achieve acceptable performance.
In the home care environment, the range of ambient and gas temperatures may well exceed that of the hospital environment. In the home care environment temperatures as low as 10° C. may be present overnight and temperatures over 20° C. may exist during the day. These temperature variations cause the commonly employed control techniques described above to suffer disadvantages. With the types of humidifiers described, condensation (or rain out) in the breathing conduit will exist, at least to some degree. The degree of condensation is strongly dependent on the ambient temperature, being much greater for greater differences between the ambient temperature and the gas temperature. The formation of large quantities of water in the breathing tubing causes considerable inconvenience to the patient, may accelerate cooling of the gas, may eventually occlude the tubing, create a gurgling sound in the tubing, or may be expelled into the patient. Also, the patient may experience discomfort when breathing gases which are delivered at temperatures widely divergent from that of the ambient temperature. Excessive condensation also results in inefficient usage of the water in the humidifying chamber of the humidifier.
An attempt to solve the problems associated with respiratory systems for home use has involved monitoring ambient temperature and air flow rate as inputs to a control algorithm which predicts corrective heater input to track the user's original setting. This approach, however, still relies on the user to determine an adequate setting for each use condition.